A VVT (Variable Valve Timing) apparatus has conventionally been known that changes the timing at which an intake valve or an exhaust valve is opened/closed, that is, the opening/closing phase (crank angle) according to an operating condition. Generally, in the variable valve timing apparatus, the phase is changed by rotating a camshaft, which opens/closes the intake valve or exhaust valve, relative to a sprocket or the like. The camshaft is rotated by an actuator such as a hydraulic or electric motor.
In an engine having a configuration with a plurality of cylinder groups (banks), the VVT mechanism is provided in each bank. Therefore, when the amount of air introduced to the cylinders differs bank by bank in such an engine configuration, variation of engine rotation (variation in rotation speed while the crankshaft rotates once) tends to occur, which may lead to larger engine vibration. Therefore, it is necessary to adjust setting of valve timings among the cylinder groups (banks).
In this respect, Japanese Patent Laying-Open No. 2003-172160 (Patent Document 1) discloses a variable valve timing control device of an internal combustion engine that enables matching of responsiveness to valve timing control among a plurality of cylinders, even if there is imbalance in camshaft load torque among the plurality of cylinders due to difference in the type of accessories. Particularly, Patent Document 1 addresses the design in which accessories such as a fuel pump (particularly a high-pressure pump supplying fuel to an in-cylinder direct injector) and a vacuum pump are driven only by the camshaft on the cylinder group of one side (bank of one side). Specifically, in consideration of a delay in valve timing control of a specific cylinder group caused by the load of accessories, the amount of control of a valve timing adjusting portion for the specific cylinder group and/or the other cylinder group is corrected so that the responsiveness to valve timing control of the specific cylinder group matches the responsiveness to valve timing control of the other cylinder group.
In order to accurately control the valve opening/closing phase (valve timing) using a variable valve timing apparatus, it is necessary to prevent error in detecting the actual phase of valve opening/closing. In order to reduce the detection error, it has been a common practice to set the valve opening/closing phase at a prescribed reference position that is limited mechanically, and to learn the error in the detected value of valve opening/closing phase at that time as an offset (see, for example, Japanese Patent Laying-Open No. 2004-156461).
In a variable valve timing apparatus disclosed in Patent Document 2 (Japanese Patent Laying-Open No. 2004-156461), reference position of the valve timing is learned under prescribed learning conditions (for example, every time engine operation starts), to ensure detection accuracy of the actual valve timing. Further, according to the disclosure, when learning is not complete, it is determined that the detection accuracy is low, and the rate of change in valve timing is limited. Consequently, damage to the apparatus caused by a movable portion hitting a stopper or the like at high speed can be prevented.
As described above, in the engine having a configuration with a plurality of cylinder groups (banks), consideration is necessary to match valve timing settings among the cylinder groups (banks) at the time of valve timing control also during such reference position learning as described in Patent Document 2. Patent Document 2, however, is silent about how to execute the reference position learning for the variable valve timing setting apparatus provided in the engine of such a configuration.